1. Field of the Invention
The present invention relates to charged-particle beam processing. In particular, the present invention relates to methods of controlling charge-up of the target of a charged-particle beam such as a focused-ion beam (FIB) or electron beam.
2. The Prior Art
Reticles and masks are used during IC manufacturing to print circuit elements and wiring on silicon. Reticles are also used in the production of masks. Each individual reticle costs on the order of $2000 and typically requires two plus weeks to make. Mask production likewise involves substantial time and expense. The complete circuit patterning for a modern IC will typically require 10 to 20 or more reticles. The ability to modify or repair a reticle or mask quickly is valuable, saving turnaround time and costs.
Focused ion beams have been used for repair of optical masks and reticles since the mid-1980s. Reticles typically consist of an opaque thin film of metal, such as chromium, deposited in a pattern on a transparent substrate of quartz or glass. Masks are of similar construction. The ability of the FIB to accurately remove unwanted portions of the metal film and to deposit material to "edit" the pattern makes it potentially an almost ideal repair tool. However, the highly-insulating surface of the substrate and the electrically-floating patterns of metal film charge-up when the FIB is applied, causing severe beam drift. FIB milling and metal deposition is also used in the semiconductor industry for analysis and repair of integrated circuits.
In some FIB systems, an electron flood gun is used to minimize the charging problem and facilitate repair. For example, U.S. Pat. No. 4,639,301 to Doherty et al. describes a FIB system in which a beam of electrons is directed on the target to neutralize the charge created by the incident ion beam. U.S. Pat. No. 4,609,809 to Yamaguchi et al. describes a FIB system in which the ion beam is neutralized by an electron shower so as to prevent the wiring portion of an IC from being charged electrically.
U.S. Pat. No. 4,503,329 to Yamaguchi et al. describes an arrangement for ion beam machining of mask defects in which electron shower units are provided for neutralizing the ion beam. Alternatively, the electron shower units are directed to the surface of the specimen to prevent charging. In another embodiment, a prober attached to a grounded arm is placed in contact with a mask pattern, for discharge to ground of charges on the pattern when the ion beam is projected onto the specimen. In a further arrangement, the entire surface of the mask substrate is coated with a thin film of metal or conductive compound such as In.sub.2 O.sub.3 or SnO.sub.2 by evaporation. In this arrangement, charges on the mask pattern can be discharged to ground through a conductive clamp which fixes the substrate in a specimen tray.
Surface charging of the target specimen is also encountered in probing and imaging with electron beam systems. With either FIB or electron-beam systems, it is known to deposit a thin conducting layer of material (usually gold or carbon) on a sample prior to imaging to prevent surface charging by providing a leakage path to ground for surface charge. The coating is evaporated onto the specimen in a deposition chamber, after which the specimen is mounted (with suitable electrical contact from the conducting layer to ground) in the vacuum chamber of the FIB or electron-beam system for imaging.
It is also known to deposit a metal film on a specimen to be sectioned with a FIB for observation. For example, U.S. Pat. No. 5,028,780 to Kaito et al. discloses use of FIB and a source of W(CO).sub.6 gas to deposit a tungsten film locally in an area of the specimen where a trench is to be cut to expose a section for observation. The deposited tungsten film serves to level the top face of the sample in the area and to avoid distortion of the section which would otherwise be caused by redeposition of sputtered material.
The use of electron beams or showers for charge neutralization has drawbacks. For example, the cost and complexity of the required FIB system is increased, and today's commercial FIB reticle repair systems with electron-flood guns can only neutralize relatively low mean beam current densities from the FIB, thus slowing the repair process.
The use of a prober in contact with a mask pattern also has drawbacks. The cost and complexity of operating a prober within the evacuated specimen chamber is a consideration. Also, a probe in contact with one portion of the pattern may not adequately discharge the surface charges on the mask if adjacent portions of the pattern and the insulative substrate are allowed to charge-up as the ion beam is applied.
Evaporating conductive coatings onto the specimen also has drawbacks. Additional equipment is required for deposition and/or removal of the coating. The coating may interfere with bonding of material to be deposited when effecting a repair, and can be difficult to remove. Typically, a carbon coat can be removed from the surface of an IC specimen with an oxygen plasma, but semiconductor manufacturers are reluctant to expose reticles to an oxygen plasma as it adds an additional complex and time consuming handling step and may degrade the optical qualities of the substrate.